Technical Field
The present disclosure relates to an ultra high density transparent flat panel display. Especially, the present disclosure relates to a transparent organic light emitting diode display having the ultra high density.
Description of the Related Art
Nowadays, various flat panel displays (or “FPD”) are developed for overcoming many drawbacks of the cathode ray tube (or “CRT”) which is heavy and bulky. The flat panel display devices include the liquid crystal display device (or “LCD”), the field emission display (or “FED”), the plasma display panel (or “PDP”), the electro-luminescence device (or “EL”) and so on.
As a self-emitting display device, the electro-luminescence device has the merits that the response speed is very fast, the brightness is very high and the view angle is large. The electro-luminescence device can be categorized an inorganic light emitting diode display and an organic light emitting diode display (or “OLED”). As having the good energy efficiencies, the lower leaked current and the easiness for representing color and brightness by current controlling, the OLED using the organic light emitting diode is more required.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer EML of the organic light emitting diode as shown in FIG. 1.
The OLED using the organic light emitting diode having the good energy efficiencies can be categorized into the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applying to the organic light emitting diode using the thin film transistor (or TFT). Hereinafter referring to FIGS. 2 and 3, we will explain about the organic light emitting diode display according to the related art.
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is a plane view illustrating the structure of the AMOLED according to the related art. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the bottom emission type AMOLED according to the related art.
Referring to FIGS. 2 and 3, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLE connected to the driving thin film transistor DT. By depositing a scan line SL, a data line DL and the driving current line VDD on a substrate, a pixel area is defined. As an organic light emitting diode is disposed within the pixel area, it defines an emission area.
The switching thin film transistor ST is formed where the scan line SL and the data line DL is crossing. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OLE disposed at the pixel selected by the switching thin film transistor ST.
The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLE. Between the anode electrode ANO and the cathode electrode CAT, an organic light emitting layer OL is disposed. The base (or low) voltage line VSS is connected to the cathode electrode CAT. A storage capacitance Cst is formed between the gate electrode DG of the driving thin film transistor DT and the driving current line VDD or between the gate electrode DG of the driving thin film transistor DT and the drain electrode DD of the driving thin film transistor DT.
Referring to FIG. 4, we will explain about the bottom emission type organic light emitting diode display. On the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the gate contact hole GH penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
The upper surface of the substrate SUB having these thin film transistors ST and DT is not in even and/or smooth conditions, but in uneven and/or rugged conditions having many steps. In order to get best light emitting efficiency, the organic light emitting layer OL would be deposited on an even or planar surface. So, to make the upper surface in planar and even conditions, the over coat layer OC is deposited on the whole surface of the substrate SUB.
Then, on the over coat layer OC, the anode electrode ANO of the organic light emitting diode OLE is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT through the pixel contact hole PH penetrating the over coat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank BN is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD, for defining the light emitting area. The exposed portion of the anode electrode ANO by the bank BN would be the light emitting area. The organic light emitting layer OL is deposited on the anode electrode ANO exposed by the bank BN. On the organic light emitting layer OL, a cathode electrode CAT is deposited.
A spacer is disposed on the substrate SUB having the cathode electrode CAT. It is preferable that the spacer is disposed on the bank BN, non-emission area. With the spacer, an en-cap is joined on the lower substrate SUB. For attaching the en-cap and the lower substrate SUB, an adhesive layer or adhesion material (not shown) would be deposited there-between.
For the bottom emission type organic light emitting diode display, the lights from the organic light emitting layer OL would be radiated to the lower substrate SUB. Therefore, it is preferable that a color filter CF is disposed between the overcoat layer OC and the passivation layer PAS and the anode electrode ANO includes a transparent conductive material. Further, the cathode electrode CAT preferably includes a metal material having the high reflection property for reflecting the lights from the organic light emitting layer OL to bottom side. In addition, the organic light emitting layer OL and the cathode electrode CAT would be deposited as covering the whole surface of the substrate.
The cathode electrode CAT is supplied with the reference voltage of the organic light emitting diode OLE. For ensuring the stable operation of the organic light emitting diode OLE, the reference voltage should be kept in stable voltage without flickers. To do so, it is preferable that the cathode electrode CAT has the low resistance metal material and is deposited over the whole surface of the substrate SUB.
When the organic light emitting diode display according to the related art is used for a long time, the video quality may be degraded due to the change of the electric characteristics of the pixels. The compensation elements for recovering these defects are required by detecting the changes of the electric characteristics.
In the cases that these compensation elements or circuits are installed into the pixel area, it may cause the reduction of the aperture ratio which is the ratio of the emission area to the pixel area. For the ultra high resolution display including UHD or 4K, the pixel area includes the switching thin film transistor, the driving thin film transistor and the compensation thin film transistor so that the aperture ration is remarkably reduced. It is required that the new structure of the organic light emitting diode display which ensures the high aperture ratio with the ultra high density resolution.
In addition, the organic light emitting diode display would be applied to the specific displays having unique function such as a transparent display in which the display images and/or information are represented with the background views of the display. However, the specifically used displays should have the more unique features than the general purpose organic light emitting diode display for realizing their specific purposes. For example, a transparent organic light emitting diode display having an ultra high density is more needed.